Magma is molten (igneous) rock formed when Earth's radioactivity heats rocks – the deeper in the Earth, the hotter it is, as diamond miners in South Africa could tell you. Under the pressure of overburden, magma can rise toward the surface through cracks in the overlying rock strata. Magma may never reach the surface, and if magma cools and solidifies in rock cracks within these strata, then it forms dikes, sills, diapirs, or huge solidified magma chambers called batholiths, any of which may later become exposed at the surface by erosion.
When melted rock flows at the surface it is called lava – Hawaii's and Iceland's basalt lavas fountain or flow freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (Mount St. Helens, Vesuvius, Pinatubo). In the distant past, flows of basalt lava were much more extensive than current flows and not only formed extensive geological provinces, but have been implicated in extinction events.
Well known flood basalts include the Columbia River Basalts formed a mere 16 million years ago and are now exposed in the Snake River Gorge in Idaho, the Siberian Flood Basalts that are believed responsible for the Earth's greatest extinction event 249 ± 1 years ago at the Permian-Triassic boundary, and the Deccan Flood Basalts in India, which formed 66 million years ago, give or take a million.) Interestingly, the Deccan Flood Basalts were formed at roughly the time of the dinosaur extinction at the boundary between the Cretaceous and Tertiary, though this extinction is widely believed (by scientists not creationists!) to have resulted from the chicxulub meteor impact.
Also of possible interest "A Diamond's Journey" on msnbc.